1. Field of the Invention
The present invention generally relates to speech recognition control systems. More particularly, the present invention relates to a hands-free voice command control system especially suited for use in a noisy environment such as encountered in vehicular radiotelephone applications.
2. DESCRIPTION OF THE PRIOR ART
Typically, in both radio and landline telephone systems, the user communicates by means of a handset that includes a speaker at one end, which is placed close to the user's ear, and a microphone at the other end, which is held close to the user's mouth. In operation, one hand of the user is occupied holding the handset in its proper orientation, thereby leaving the user's only free hand to accomplish tasks such as driving a vehicle. In order to provide a greater degree of freedom for the user, speakerphones have commonly been used in landline telephone systems. Recently, vehicular speakerphones have been developed for use in automobiles. For example, U.S. Pat. No. 4,378,603 by Eastmond and U.S. Pat. No. 4,400,584 by Vilmur, both assigned to the same assignee as the present invention, describe vehicular speakerphones with hands-free operation.
Hands-free control systems, which are responsive to human voice, are highly desirable for use in automotive applications. Most mobile radiotelephone transceiver functions (e.g., on/off, transmit/receive, volume, squelch, etc.) or mobile radiotelephone control functions (e.g., pushbutton dialing, speech recognizer training, telephone call answering) may readily be achieved by voice command without requiring any manual operations. Hence, the use of speech recognition with a vehicular speakerphone has the potential for providing a totally hands-free telephone conversation without ever requiring the automobile driver to remove his hands from the steering wheel or take his eyes off the road. This feature has not only extended the hands-free capability, but also has added to the safety and convenience of using mobile radiotelephones in vehicles.
Unfortunately, the application of complete hands-free control to a mobile radiotelephone introduces several significant obstacles. When speech recognition is utilized in a vehicular environment, the high degree of ambient noise inherent in a vehicle presents a considerable problem to reliable voice control. Furthermore, a vehicular speakerphone typically has a microphone that is distant from the user's mouth, such as being mounted overhead on the automobile sun visor. Consequently, the required high microphone sensitivity causes a large increase in the amount of environmental background noise being applied to the speech recognizer, which degrades its recognition reliability.
More significantly, however, is the problem of speech recognition falsing. If the voice command controller erroneously recognizes a "call answering" command, the speech communications device will simply come "off-hook" at a time when it is not desired -- which is typically only a minor inconvenience. However, if the controller erroneously detects a "call terminating" command, the controller will put the device "on-hook" and the telephone call will be inadvertently dropped -- which is, at the very least, considerably frustrating to the user. Hence, much more significant consequences result from speech recognition falsing on the call terminating command than on other command words.
For these reasons, the prior art techniques for terminating a hands-free telephone conversation have been severely limited. Most systems are not totally hands-free, since phone calls must still be answered and terminated manually. For example, even though cellular mobile radiotelephones may provide a voice command dialing feature, they often require the user to pick up the handset to activate the voice command system. Other vehicular telephone systems require the user to activate a "call terminate" button mounted on the steering wheel. Still other voice-controlled telephone systems disconnect the call upon the recognition of a voice command corresponding to a single isolated word speech recognition template. In a high noise environment, there is a substantial likelihood that a speech recognizer may false and improperly drop the call. Moreover, a call may be disconnected if the mobile user inadvertently mentions the voice command word during his conversation. The chance of falsing is further increased with the desirable feature of speech recognizer robustness to variability in user voice characteristics.
A need, therefore, exists for a reliable method for terminating a telephone call by voice command in a hands-free speech communications device.